The present invention involves an improved chopper for chopping continuous or very long loose items such as fiber, fiber strands, yarn, wire, string, ribbon, tape and the like by pulling the item(s) into the chopper while the loose items are held tightly against the surface of a rotating backup roll and carrying the item(s) on into a nip between a rotating blade roll and the rotating backup roll where they are separated into short pieces. More specifically the present invention involves a chopper having an improved backup roll and methods of making and using the improved backup roll.
It has long been known to chop continuous fibers or fiber strands into short lengths of about 5 inches or shorter. Billions of pounds of such product including chopped glass fibers and fiber strands are produced each year in process and chopping apparatus such as disclosed in U.S. Pat. Nos. 5,970,837, 4,398,934, 3,508,461, and 3,869,268, the disclosures of which are incorporated herein by reference. The choppers disclosed in these patents comprise a blade roll containing a plurality of spaced apart blades for separating the fibers into short lengths, a backup roll, often driven, having a working surface that the blades work against to effect the separation, and that also pulls the fibers or fiber strands. In the processes disclosed in these patents, the chopper is usually the most productivity limiting equipment in the processes. These processes typically operate continuously every day of the year, 24 hours each day, except during furnace rebuilds every few years. Therefore, improvements in the chopper, that allow the chopper to pull and chop faster and for longer times between maintenance shutdowns, and to have shorter duration shutdowns for maintenance have an extremely positive impact on productivity and production costs.
The working surface layer of the backup roll is a somewhat soft material that starts out about two inches thick. During operation the surface of the working surface layer becomes rough because of the blades penetrating the surface repeatedly to break or chop the strands of fibers into desired lengths. When the surface roughness becomes too severe, some of the fibers in the strands are not chopped and this produces double cuts, long cuts, uncuts, etc., i.e. fibers longer than the desired length and multiples of the spacing between the blades, which is undesirable and causes scrap and defects in the products the fibers are used to make, such as nonwoven webs, composites, etc. Prior to such a condition developing to a costly extent, the surface of the working surface layer is ground down in place on the chopper, or the backup roll, or working surface layer are removed and ground down off-line.
It has been typical to remove the entire backup roll to do this, but more recently it has also been taught to remove only the working surface layer, see U.S. Pat. No. 6,619,573. The backup rolls are very heavy on most choppers requiring two workers and lifting aids, or one worker and a precise lifting aid to change the backup roll quickly, i.e. within 5-10 minutes. The fiber forming rooms where the choppers operate are typically crowded and it is not practical or desirable to have to bring in bulky equipment to provide lifting aid for a new or reconditioned backup roll. The working surface layer is much lighter and can easily be lifted and placed by one worker. To reduce downtime significantly it is taught in U.S. Pat. No. 6,619,573 to use a collapsible mandrel as the wheel for the backup roll and to replace only the working surface layer. This solution works well, but requires replacement of all the existing wheels with a collapsible wheel that is more costly than a standard wheel.
Normally several strands such as up to 14 are fed into the chopper, each strand containing 2000 or more fibers. As more fiber strands and fibers are fed into the chopper it becomes more difficult to pull all of the strands and fibers at the same speed, so more pressure is applied to the cylinder pushing the idler roll against the backup roll with more force.